feat: default tracker configs, email validation, expandable target links

- Tracker now has default_tracking_config_id and default_template_config_id
  that apply to all linked targets unless overridden per-target
- Dispatch falls back to tracker defaults when per-link configs are null
- Email bot creation validates SMTP connection before saving
- Email notifications sent as HTML (links render properly)
- Linked target items are expandable: collapsed shows config CrossLinks,
  expanded shows config selectors; action buttons always visible
- Fix email bot test button icon (mdiEmailSend → mdiSend)
- Fix target type icons in LinkedTargetsSection for all types
- Provider filter moved above search in sidebar

packages/server/src/notify_bridge_server/database/models.py

@@ -309,6 +309,8 @@ class NotificationTracker(SQLModel, table=True):
     filters: dict[str, Any] = Field(default_factory=dict, sa_column=Column(JSON))
     scan_interval: int = Field(default=60)
     batch_duration: int = Field(default=0)  # seconds to accumulate events before dispatch (0=immediate)
+    default_tracking_config_id: int | None = Field(default=None, foreign_key="tracking_config.id")
+    default_template_config_id: int | None = Field(default=None, foreign_key="template_config.id")
     enabled: bool = Field(default=True)
     created_at: datetime = Field(default_factory=_utcnow)